The invention concerns scanning of originals, such as letters and drawings, where the information on the original is entered optically with a scanner. Subsequently an electric representation is generated for the entered original.
Scanners of this type are widely known. With respect to the quality of a reproduced image it is important that the electric representation of the data entered by the scanner is optimum with respect to resolution and reproduction of details. The entered data are usually grey-level data, a point on the original being represented by its grey-level. The entered grey-level data are preferably digital, and an 8-bit representation (256 grey-level values) usually gives a suitable tone resolution. When subsequently reproduced, the original is usually represented by a binary representation (black/white) for each pixel. A threshold value is therefore introduced so that the electric representation of the original is reduced to comprise just a binary representation in the form of two states for each image element. The threshold value is determined from a wish to maintain as much information as possible in the image, and the threshold value is usually determined statistically in response to the distribution of the grey-level values. U.S. Pat. No. 4,972,273 is an example of such a technique.
A large number of techniques for determining threshold values for scanners are known. In some techniques an operator enters a threshold value for an original based on experience. In other systems a threshold value is determined statistically for an original by a prescan of the original, the threshold value being used in the actual scanning in connection with the image processing procedure.
However, the fact is that an original for scanning frequently has an uneven distribution of dark and light areas. Thus, a threshold value determined for the entire original may frequently cause details to disappear when the threshold value is introduced.
It has thus been attempted to develop techniques for dividing the original into zones to determine threshold values for the individual zones. The information of the original is entered linewise so that it has been necessary to enter and store the original in its entirety and subsequently determine the threshold values statistically. This technique is vitiated by a very important drawback--viz. storage of large data amounts. In case of a large original in e.g. AO format (36.times.44 inches) and with a resolution of 300 dots per inch, the stored data amount corresponds to about 143 Mbytes, since each pixel contains grey-level information corresponding to typically 8 bits. Thus, very large data amounts are to be processed merely to calculate the threshold value which is subsequently to be used for the actual data processing. Following this the individual pixels are to be compared with respective threshold values.
The object of the invention is to provide a method of scanning an original in a manner that allows for variations in the information across the surface of the original, without this requiring storage of large data amounts.